Separation of m-phenylphenol and p-phenylphenol has posed a serious problem in preparing resin grade p-phenylphenol. Separation of o-phenylphenol from these two isomers is normally accomplished by distillation, but the m- and p-isomers cannot be separated in this manner because of the close boiling points.
In the past, these two isomers have been separated by two methods. The first method is described in U.S. Pat. No. 2,129,908 whereby m-phenylphenol is separated from a mixture of m- and p-phenylphenols either by leaching out the meta compound from the mixture with an organic solvent at a temperature at which the para compound is substantially insoluble therein or by dissolving the aforesaid phenylphenol mixture in a heated organic solvent and then cooling the solution to precipitate the para compound. The patent goes on to say that the p-phenylphenol is thus partially purified (containing about 5% of m-phenylphenol) and that this can be further purified by using hot caustic to dissolve all of the phenylphenol. The hot solution is then cooled whereby crystallization of the sodium p-phenylphenate occurs.
A second method used in industry borrows in part from the above. That is to say, technical grade or crude p-phenylphenol containing an undesirable amount of m-phenylphenol is entirely dissolved in excess caustic. This reaction produces sodium m-phenylphenate and sodium p-phenylphenate, the cooled solution producing crystallization of the sodium p-phenylphenate. These crystals are then filtered off, washed with cold water and redissolved in boiling water. Resultant solution is then acidified with dilute sulfuric acid whereby p-phenylphenol is precipitated. The disadvantage of this process is that there are two waste streams that have to be sent to waste disposal.
Due to the great expense of these techniques, research has been conducted to solve this problem. The present invention is a result of such research.